There has been known a coated separator (laminated porous film) producing method in which a belt-like porous film that is transferred by a transfer system including a plurality of transfer rollers is coated with a coating solution while being transferred (Patent Literature 1).
Patent Literature 1 shows the following knowledge. Specifically, a longer distance between two transfer rollers that are adjacent to each other in a direction in which a battery separator is transferred (hereinafter also referred to as a “roller span”) causes the battery separator to easily have a wrinkle in one of the two adjacent transfer rollers which one is located on the downstream side in the direction in which the battery separator is transferred. Thus, any adjacent transfer rollers of the transfer system preferably have a roller span of not more than 1 m.